A1=-3 a61=57 n=15 Sn-? d-?

Question

A1=-3
a61=57
n=15
Sn-?
d-?

Answer 1

А1=-3,а61=57,п=15
а61=а1+60d
d=(57-(-3)):60=1
a15=-3+14*1=11
S=(-3+11)*15:2=60